Role of process parameters on bondability and pad damage indicators in copper ball bonding I. Qin a , A. Shah b, * , C. Huynh a , M. Meyer a , M. Mayer b , Y. Zhou b a Kulicke and Soffa Industries Inc., Fort Washington, PA, USA b Microjoining Laboratory, University of Waterloo, Waterloo, ON, Canada article info Article history: Received 17 February 2010 Received in revised form 6 April 2010 Accepted 7 April 2010 Available online 6 May 2010 abstract Cu wire bonding is one of the hottest trends in electronic packaging due to the cost and performance advantages of Cu wire over Au wire. However, there are many challenges to Cu wire bonding, one of which is the increased stress transmitted to the bond pad during bonding. This high stress is not desirable as it leads to pad damage or cratering in the Si under the pad. Another issue is pad splash in which the pad material is squeezed outside the bonded area, which in severe cases can cause Al pad thinning and deple- tion. To study the root cause of the increased stress, ball bonding is performed with Au and Cu wires using the same levels of ultrasound (USG), bonding force (BF), and impact force (IF). The bonding is performed on a bonding test pad with integrated piezoresistive microsensors and the in situ pad stress is measured in real time. The ultrasonic pad stress did not show any significant difference between the Au and the Cu ball bonding processes. This indicates that the cause of increased stress cannot be attributed to material properties such as hardness alone, and that the differences in bondability and bonding parameters required for the Cu process might be more influential. To achieve optimal bonding results in terms of shear force per unit area, the Cu process requires higher BF and USG settings, which are the main causes of pad damage. To understand the effect of bonding parameters IF, BF, and USG on pad stress, a detailed DOE is conducted with Cu wire. In addition to conventional bonding parameters, the effect of a non-zero USG level applied during the impact portion of the bonding (pre-bleed USG) is investigated. One of the findings is the reduction of pad damage when higher pre-bleed USG levels are used. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Thermosonic Au ball bonding has been the most preferred method for electrical connections to integrated circuits (ICs) [1– 3]. In this process, a thin wire is welded to a metallization pad of an IC and to a substrate terminal, thereby interconnecting the IC with a larger scale substrate circuit. However, with the skyrocket- ing price of Au, the industry is continuously looking for a cheaper alternative to Au wire. Among the alternatives being developed, Cu wire is being increasingly used in the industry [4], followed by Pd coated Cu wire [5]. Other wire materials, in particular Au–Ag wire [6] have also been developed, however, it has not been widely adopted in production. The main reason for using Cu wire instead of Au is the cost saving: at the current reference Au price of $1100– $1200 per ounce, a 500 m spool of 20 lm Au wire costs $200, which is about 10 times the cost of a comparable Cu wire. More- over, Cu has superior electrical and thermal conductivities as well as higher mechanical strength [7–10]. The higher strength of Cu al- lows for longer distances between the ball and the crescent bonds because the wire loop is more resistant to deformation. However, there are a set of challenges that need to be solved be- fore a robust Cu wire bonding process can be implemented in the industry. The first challenge is that Cu readily forms an oxide layer on its surface, which reduces its bondability. In order to limit the oxidation of Cu, the bonder must be retrofitted with a Cu kit that consists of a means of supplying a shielding gas during free air ball (FAB) formation process. The most common type of shielding gas used in Cu ball bonding is a homogeneous mixture of 95% N 2 and 5% H 2 [11]. The second challenge faced by Cu wire bonding is the narrow crescent bond process window. In particular, the short tail problems in Cu ball bonding caused by a weak tail bond [11] cause frequent production stops, lowering the mean-time between as- sists (MTBA). The third challenge is the increased risk of damage to the semiconductor chip due to the high hardness of Cu. Due to its higher hardness compared to Au wire, higher normal and ultra- sonic forces are often used in Cu ball bonding, resulting in 30% higher bonding stress [12] acting at the bond pad. The higher pad stress increases the likelihood of chip damage such as pad cracking [13,14], pad splash [15–17], silicon cratering [18–20]. 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.04.001 * Corresponding author. Tel.: +1 5198884567x33326. E-mail address: ashah011@engmail.uwaterloo.ca (A. Shah). Microelectronics Reliability 51 (2011) 60–66 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel